The car was warmed up to 90°C this morning.

Hit standstill traffic, after 10 minutes or so the temperature started dropping to 60°C and seems it would have dropped further if standing still.

Once driving it reached the normal 90°C

Is this normal? Maybe the engine is just too efficient?

It was -9°C to -5°C outside

Does it fluctuate often?

It not, and this is what happens in low temps, then thermostat is likely culprit.

Wouldn't worry about it - you'd struggle to get the car to warm up at all from cold in this weather, so I imagine a prolonged spell in a traffic jam would lose more heat through the radiator than the engine would be producing at idle.

Normal in these conditions. Diesels produce very little heat compared to petrol cars.

Wouldn't worry about it - you'd struggle to get the car to warm up at all from cold in this weather, so I imagine a prolonged spell in a traffic jam would lose more heat through the radiator than the engine would be producing at idle.

indeed, I took the punto out recently, and had the heater on, with very gentle driving, an hour and it never got to 'normal' temp! the air was so cold going through the heater matrix, it cooled the engine sufficiently (it was warm in the cabin) , I don't beleive it even opened the valve to the radiator! lol...